This invention relates to filtration and, more particularly, to the removal of submicron contaminants from aqueous systems, utilizing filter media sheet comprising high levels of particulate filter aids.
The filtration of fine particle size contaminants from fluids has been accomplished by the use of various porous filter media through which the contaminated fluid is passed. To function as a filter, the media must allow the fluid, commonly water, through, while holding back the particulate contaminant. This holding back of the contaminant is accomplished by virtue of the operation, within the porous media, of one or both of two distinctly different filtration mechanisms, namely (1) mechanical straining and (2) electrokinetic particle capture. In mechanical straining, a particle is removed by physical entrapment when it attempts to pass through a pore smaller than itself. In the case of the electrokinetic capture mechanisms, the particle colloides with a surface face within the porour filter media and is retained on the surface by short range attractive forces.
With the exception of microporous polymeric membranes, the porous filter media known to the art as being suitable for the filtration of fine particle size contaminants are comprised of fiber-fiber or fiber-particulate mixtures formed dynamically into sheet by vacuum felting from an aqueous slurry and then subsequently drying the finished sheet. In those fibrous filter media that depend upon mechanical straining to hold back particulate contaminants, it is necessary that the pore size of the filter medium be smaller than the particle size of the contaminant to be removed from the fluid. For removal of fine, submicronic contaminant particles by mechanical straining, the filter media need have correspondingly fine pores. Since the pore size of such a sheet is determined predominantly by the size and morphology of the materials used to form the sheet, it is necessary that one or more of the component materials be of a very small size, such as small diameter fibers. See, for example, any of Pall U.S. Pat. Nos. 3,158,532; 3,238,056; 3,246,767; 3,353,682 or 3,573,158.
As the size of the contaminants sought to be removed by filtration decreases, especially into the submicron range, the difficulty and expense of providing suitably dimensioned fiber structures for optimum filtration by mechanical straining increases. Accordingly, there is considerable interest in the use of fine particulates such as diatomaceous earth.
However, for such materials it is necessary to provide a matrix in order to present a coherent handleable structure for commerce and industry. Thus, at least one of the component materials in the sheet is a long, self-bonding structural fiber, to give the sheet sufficient structural integrity in both the wet "as formed" and in the final dried condition, to allow handling during processing and suitability for the intended end use. Unrefined cellulose fibers such as wood pulp, cotton, cellulose acetate or rayon are commonly used. These fibers are typically relatively large, with commercially available diameters in the range of six to sixty micrometers. Wood pulp, most often used because of its low relative cost, has fiber diameters ranging from fifteen to twenty-five micrometers, and fiber lengths of about 0.85 to about 6.5 mm.
Filter media sheets are conveniently formed by vacuum felting from an aqueous slurry of the component material. The vacuum felting is performed on a foraminous surface, normally a woven wire mesh which, in practice, may vary from 50 mesh to 200 mesh, with mesh openings ranging from 200 micrometers to 70 micrometers respectively. Finer meshes are unsuitable because of clogging problems and/or structural inadequacy.
The size of the openings in the foraminous vaccum felting surface, and the pore size of the cellulose fiber matrix of the formed sheet, are quite large in comparison to some or all of the dimensions of the fine fiber or particulate components required to produce the desired submicronic filter media sheet. Retention of such fine components during the vacuum formation of the filter media sheet is difficult, and imposes severe constraints on the choice of such materials, the specific details of the process utilized to form the filter media sheet, and, most important, upon the level of filtration performance that may be attained. Fine fibers, whose length may be large in comparison to their diameter, present less of a problem and tend to be retained reasonably well. Fine particulates, on the other hand, tend to show very poor retention during sheet formation.
Flocculation with polymeric retention aids, or coagulation has been used as a means of improving retention of fine particulates, in effecting the grouping of particles to offer an effective large dimension. However, filter sheet prepared from a well-flocculated slurry will have a broad particle size distribution, with small pores occurring inside the flocs, and large pores occurring between the flocs. The existence of these larger pores will limit the ability of the filter media sheet to remove fine contaminants. The use of flocculation to achieve high retention in filter media is therefore somewhat counterproductive.
It is, of course, possible to apply hydrodynamic shear forces, breaking up the flocs, and further charge modify until the system assumes a stable disperse form. This does achieve a relatively uniform sheet of narrow pore size distribution. However, the retention of the particulates in such a system is very low, leading to concomitant reduction of filtration efficiency.
In addition to controlling the dispersion characteristics (and therefore the porosity of the sheet) and providing wet strength, charge modifiers are employed to control the zeta potential of the sheet constituents and maximize performance in the electrokinetic capture of small charged contaminants. In practice, cationic charge modifiers are employed since most naturally occurring contaminant surfaces are anionic at fluid pH of practical interest. A preferred charge modifier is a melamine-formaldehyde colloid disclosed for filter sheets in U.S. Pat. Nos. 4,007,113-4.
Generally, filter media employing charge modified elements are utilized without further treatment after media formation. However, where treatment of biological liquids is contemplated the filter media are commonly subjected to sanitization or sterilization procedures under stringent conditions, e.g., relatively high temperatures and pressures, and these conditions can effect some diminution of performance with certain charge modified media or increase extractables beyond allowable limits. This phenomenon although not entirely understood may be attributable to loss of or degradation of the resin itself, or the charge functionality from the system through chemical or physical action. In any event, it appears to be specific to the choice of resin. In particular, the melamine-formaldehyde colloid modified system is incapable of satisfactorily resisting autoclaving or hot water flush conditions, losing significant amounts of charge, thus limiting its effective life. This charge modifier, being of low molecular weight, also is limited in respect to the amount of charge modification which can be achieved in its use. Substitutes to meet these limitations are of particular interest.
It is accordingly an object of the present invention to provide charge modified filter media sheets of enhanced filtration performance, especially for the removal of submicron contaminants from aqueous systems at high efficiency.
Another object is to afford a filter media sheet comprising a high level of fine particulates.
A still further object is the provision of sanitizable or sterilizable filter media resisting degradation in such treatment and retaining sufficient charge potential to effectuate enhanced electrokinetic capture of small charged contaminants.
These and other objects are achieved in the practice of the present invention as described hereinafter.